Production of metal halide ingots



y 16, 1951 R. A. LEFEVER 2,984,626

PRODUCTION OF METAL HALIDE INGOTS Filed 001;. 16, 1956 INVENTOR ROBERTA. LEFEVER A T TORNE V United States Patent PRODUCTION OF METAL HALIDEINGOTS Robert Lefever, Bon Air, Va., assignor to Union CarbideCorporation, a corporation of New York Filed Oct. 16, 1956, Ser. No.616,170

8 Claims. (Cl. 252-3014) This invention relates to an improved processfor the production of large ingots of fusible materials, particularly ofcertain metal halides with or without small quantities of activators.Transparent masses are used to construct prisms and windows for use inspectroscopy, for example, in infra-red analyzers. When such ingotscontain an activator, they are also useful for detecting and measuringradiation energy.

Ingots of certain metal halides, for example, the alkali metal group,are widely used as prisms for infra-red spectrometers, mainly becauseglass is only useful for wave lengths up to approximately 2.5 micronsand the various alkali halides are suitable for transmission ofinfra-red wave lengths as high as 30 microns, each metal halide beingadaptable for a special range. Furthermore, these materials may beconverted into phosphors by the inclusion of small quantities ofactivators such as thallium which excite the absorbed light tofluorescence and phosphorescence, and form long wave-length absorptionpeaks. These phosphor ingots may be used in scintillationcounters forthe detection of gamma rays.

Large ingots of metal halides have been produced for many years byseveral processes. For example, one process involves the slow passage ofa container with the raw constituents at a controlled rate through a hotzoneto fuse the charge. As the lower portion of the container emergesfrom the hot zone and cools, solidification initiates and graduallyprogresses up through the fused melt. As the ingot is grown, the solidis maintained at a temperature only slightly below its melting point toavoid spontaneous nucleation in the melt and the formation of voids inthe ingot. This necessitates a slow growth rate to allow sufficient timefor dissipation of the heat of solidification. Furthermore, only alimited part of the available container surface is utilized to conductheat away from the melt since duringmost of the solidification stage,asubstantial part of the container is still in the hot zone.

.In another prior art process, the raw constituents are placed in ahemispherical bowl and heated substantially above the melting point ofthe charge. The temperature isthen allowed to slowly decrease withmaximum heat dissipation at the bottom of the bowl. When the temperatureat the bottom reaches the solidification point of the charge,solidification or crystallization occurs and the solid-liquid interfacegradually progresses upwardly to the top surface. As in the firstmentioned process, the cooling rate must be low to avoid spontaneousnucleation. To increase the solidification rate, it would be necessaryto provide a high cooling rate at the bottom of the container and alsoprovide a heat source to prevent s'pontaneousnucleation in the melt. Insuch an arrangement, there is again the problem of utilizing only alimited portion of the available container surface for conducting heataway from the melt. In principle, this process is similar to. the firstmentioned prior art process except that in'the latter the container isheld stationary ice crystalline masses as large as 2 inches in diameterand 2 inches long. In addition, the ingot growth rates are low. Thefirst mentioned prior art process, for example, normally employs ratesof 1-5 millimeters of linear growth per hour.

A further disadvantage of these processes when used for producingactivated metal halide ingots, for example, sodium iodide ingotsactivated with thallium iodide, is uneven distribution of the activatorwhich in turn reduces the ingots efiiciency as a scintillation counter.This problem may be illustrated by reference to the sodiumiodide-thallium iodide system. As the ingot is grown, thallium iodide isejected from the solid into the melt thus increasing the thalliumconcentration of the melt. The distribution coeflicient for thalliumiodide between the ingot and the melt is about 0.2. This means that theinitial thallium iodide concentration in the melt must be at least 0.5weight percent to provide the minimum amount of thallium iodide (0.1percent) in the ingot required for maximum pulse amplitude or generatedsignal in the scintillation counter. As the thallium iodide is ejectedinto the quiescent melt during the prior art growth processes, it mustbe given suificient time to diffuse away from the solid-melt interfaceto prevent engulfment by the growing ingot. This places a limitingcondition on the rate of growth and introduces the undesirablepossibility of high local and general thallium concentration regions dueto growth rate fluctuations.

Another important requirement in efficient production of ingots isadequate heat removal from the ingot during the growing step. If theamount of heat withdrawn from the ingot is small, the heat input to themelt and thus the temperature ofthe melt must be kept relatively low tomaintain ingot growth. As the temperature of the melt falls towards themelting point of the charge, the quantity of melt approaching thecrystallization temperature at the solid-liquid interface graduallyincreases. If the overall temperature of the melt is reduced to a leveltoo close to the melting point, spontaneous nucleation occurs near theinterface. The resulting crystals attach to the interface, trappingliquid and eventually causing voids during contraction andsolidification. In the case of sodium iodide ingots activated withthallium iodide, the voids lead to light scattering and reducedefliciency of the material as a scintillation counter.

A still further disadvantage of the previously described well-knowningot growing processes is the occurrence of high internal strain duringthe final cooling step. If the ingot is grown in a container, the outeringot surface is in direct contact with the container. Due to thedifference in coefiicients of thermal expansion between the containerand the ingot, internal strain occurs and, if sufficiently high, theingot will crack.

Principal objects of the present invention are: to provide an improvedprocess for the production of large ingots of fusible materials,particularly of metal halides with or without small quantities ofactivators, process providing substantially higher ingot growth ratesthan heretofore obtained, and a process for the production of activatedmetal halide ingots in which the distribution of the activator issubstantially uniform throughout the ingot.

In accordance with one aspect of the present invention, a process isprovided for producing metal halide ingots in which the raw constituentsare first heated in a container to an elevated temperature above themelting 3 points of such raw constituents to form a fused melt. Thefused state is then maintained and the melt is agitated. Next, the fusedmelt is gradually cooled and the ingot is grown up a dl mt b om p ti ntoward he p ofsaid 'melt; Ihthe preferred embodiment, heat is intro cedn a nau h z ta ban a u h umference of the raw constituted charge nearthe top surface of such charge during the crystallization step. Also,the rate'of crystal growth may be controlled by varying the heat inputand the agitation of the melt, the former being preferred,Internalingotstrain during the cooling step ma 9? m nimized by heatingthe ingot sufficiently to melt at large part of the outer surface of theingot in direct contact with the container wall, and removing thismelted part from the container to leave a vacant space between thecontainer'walls and the ingot except for the small remaining unmeltedpart. The novel apparatus required for the process of this invention isdescribed in the ensuing portion of the specification,

Although the invention will be further described in terms of" growingsodium iodide ingots activated with thallium iodide, it is alsoapplicable to growth of other metal halide ingots such as the alkalimetal halides; that is, halides .of lithium, sodium, potassium, rubidiumand cesium. The process of the present invention are equally suitablefor growth of other metal halides such as thallium bromide-iodide andcalcium fluoride. Small quantities of activators may be added to theseraw constituent compounds for growth of phosphor ingots.

By agitating the melt, heat already introduced in the system during thefusing stage is uniformly distributed. When cooling is initiated, theagitation continues to uniformly distribute the available heat throughthe melt so as to prevent the occurrence of cold spots and resultantspontaneous nucleation. In the previously described prior art processes,the melt was not agitated during the fusing or the crystallizationstages and the heat was necessarily transferred to and from the inner orcentral portion of'the melt through the surrounding ring or outerportion of the melt. Only the outer portion of the melt was in directheat exchange contact with the container surface, and the heat transferwas limited by the thermal conductivity of the charge. In the presentinvention, agitation exposes the entire body of melt to the containersurface thus shortening the heat transfer path and increasing theaverage temperature difference between the container wall and the meltdirectly in contact with such wall. In this manner, agitation of themelt permits the attainment of remarkably high ingot growth rates. Forexample, ingot growth rates of 1-5 millimeters per hour have beenconsidered excellent by the prior art, and commercially attractiveingots of sodium iodide have been grown by the present invention atrates of 25-100 millimeters linear growth per hour.

The agitation technique also facilitates the rapid growth of large metalhalide crystalline masses containing sm qua i ies of uniformlydistributed activating material. For example, thallium iodide activatedsodium iodide ingots have been grown at remarkably high rates of 75millimeters per hour utilizing the novel process of the presentinvention. Substantially uniform distribution of thallium iodide isinitially obtained during the fusing step and maintained during theinitial cooling and crystallizing step by the aforementioned agitation.The prior art has established that the most desirable thallium iodideconcentration in sodium iodide phosphor ingots is about 0.1 weightpercent. As previously discussed, this requires a concentration of about0.5 weight percent in the melt. One could employ up to about 2 percentthallium iodide in the melt and obtain a satisfactory phosphor ingot,but still higher concentrations would tend to produce cloudy ingotswhich are generally unsatisfactory for spectroscopic applications.

During the crystallization step, a temperature gradient is establishedbetween the bottom and the top of the melt and crystallization isinitiated in the cooler bottom portion. The ingot is then grown from thebottom toward the top of the melt, and control of the growth rate isreadily achieved by varying the heat input or the agitation rate, or acombination of the two. Control of the heat input is the preferred meansof controlling the ingot growth rate as decreased stirring tends topermit cold spots and uneven distribution of the activatorthe problemsthat agitation is intended to overcome.

A novel feature of the preferred embodiment of the present invention isthe introduction of heat in a narrow horizontal band around thecontainer circumference near the top layer of the charge. By agitation,the heat so introduced is uniformly distributed throughout the charge aspreviously described, and during the crystallization step sufficientheat to avoid spontaneous nucleation is provided by this method. Byintroducing the heat in a narrow band, the maximum available containersurface is utilized for radiation and conduction of heat away from themelt during the crystallization step. This arrangement provides anothermethod of increasing the ingot growth rate.

A further novel feature of the present invention is the avoidance ofexcessive internal strain during the cooling step following growth ofthe ingot. This is achieved by first heating the ingot sufliciently tomelt a large part of the outer surface of the ingot in direct contactwith the container, and then removing this melted outer surface leavinga vacant space between the container walls and the ingot except for thesmall remaining unmelted part of the outer surface supporting the ingot.The ingot is then reheated sufiiciently to obtain a substantially uniform temperature throughout the mass. When the ingot is subsequentlycooled, the vacant space permits thermal contraction with minimum strainbecause of the small contact area between the ingot and the container.The cooled ingot may easily be removed from the container by severingthe small remaining unmelted outer surface of the ingot.

The drawing shows a vertical longitudinal section of one embodiment ofthe novel apparatus for producing ingots by the process of the presentinvention. The container 10 holding the charge 11 is enclosed by a cap12 at its upper or top end. The container 10 and cap 12 are constructedof heat-resistant material which is thermally stable above the fusiontemperature of the charge and which does not shed impurities into thecharge 11. A concentric ring burner 13 is placed around the container 10with ports 14 therein for the passage of a flame 15. These holes 14 arepreferably uniformly spaced around the circumference of the ring burner13 to provide uniform distribution of heat around the containercircumference in a narrow horizontal band 21. Agitation is facilitatedby a disk 16 moving in a longitudinal direction, up and down, on the endof a rod 17 passing through a port 18 in the container cap 12. The rodmay be actuated manually, but preferably is oscillated by powertransmitted from a small electric motor. The preferred amplitude androd-disk stroke rate depend on the type of ingot to be grown and thespecific apparatus used for such growth. I have successfully growningots by the process and apparatus of the present invention usingamplitudes of inch to inch with stroke rates of 60 to 200 per minute.

Agitation may be started during the fusing step as soon as the charge 11has melted sutficiently to permit movement of the disk 16 therein, butagitation is not essential until the entire charge has melted or fusedand the crystallizationstep is to be initiated. Either or both theamplitude and the position of the rod and disk assembly are controlledduring the crystallization step so that effectiveagitation isobtainedwithout allowing the disk to actually contactthe upwardly growingsolid-liquid interface nor so agitating the top surface of .the meltthatgas bubbles are entrained and carried down to the growing ingot. Whenproducing ingots of oxygenesensitive or moisture,-

ifi im l for ex mp S d um d e fluoride, potassium iodide, and calciumfluoride, an inert atmosphere may be maintained in the container toavoid oxidation or hydrolysis and resultant contamination of the charge.An inert gas such as argon is introduced through conduit 19, thelatterpassing through a port 20 in the container cap. 12. The inert gasis discharged through the port 18 in the container cap 12 to avoidpressure buildup in the container 10. In this manner, the charge isblanketed fiom the atmosphere by a continuous flow of circulating inertgas.

The following example describes the subject process in operation:EXAMPLE Preparation of sodium iodide ingots activated by thallium iodideAbout 150480 grams of sodium iodide and 0.75-0.9 gram of thallium iodidewere placed in a 1% inch outside diameter by 1% inch inside diameterfused silica container similar to that illustrated in the drawing. Thecharge was fused under a continuous flow of argon gas. Rapid verticalagitation, approximately 200 strokes per minute, was maintained by useof a fused silica disk (about /2 inch in diameter) moving up and down onthe end of a fused silica rod (Mi-V2 inch'stroke) by power transmittedfrom a small electrical mctor. Sufficient heat was supplied by means ofa ring burner placed concentrically around the container, and the flamewas directed in a narrow horizontal band near the top of the charge tomaintain such charge in the molten state. The melt temperature duringthis step was in the range of 690 C.- 750 C., the melting point ofsodium-thallium iodide mix being 660 C. Next, the stirring rate wasslowly decreased uutil solidification began at the bottom of thecontainer. By gradually decreasing the stirring rate and the heatsupplied by the burner, the solid-liquid interface was made to progresstoward the burner. At the same time, the liquid level gradually lowereddue to contraction of sodium iodide on solidification. Growth proceededat the rate of about one inch (25.4 millimeters) per hour. After growthof about 1% to 1 /2 inch length of solid ingot, the container wasinverted and the remaining molten liquid was poured oflF. The outersurface of the ingot in direct contact with the container was thenmelted away except for a small connection so that the ingot could notdrop out. This greatly reduced strain on the ingot during furthercooling, and prevented cracking. The container and the ingot were thenplaced in a furnace at about 650 C. and slowly cooled to roomtemperature during a period of about 4-5 hours to minimize internalthermal stress and avoid cleavage.

Although the preferred embodiment of the invention has been described indetail, it is contemplated that modifications of the process maybe madeand that some features may be employed without others, all within thespirit of the invention and the scope thereof as set forth in theclaims. For example, although a rod and disk stirring arrangement hasbeen described, other forms of agitation might also be employed, such asbubbling an inert gas through the melt, or rotating mixers.

Another possible modification is control of the crystal growth rate bythe regulation of the heat loss through the lower part of the container;that is, forcibly cooling the portion of the container containing thegrowing ingot. However, as previously discussed, the preferred method ofcontrolling the ingot growth rate is by control of the heat input.

A further variation is that of fusing the charge and growing the ingotsin dilferent containers. Such an arrangement is adaptable to massproduction techniques by, for example, fusing the charge in a largesingle container and then transferring the molten mass to a battery ofsmaller containers for ingot growth therein. In the case of growingphosphor ingots, the additive could either be injected in the largesingle container or in the battery of smaller containers.

. v "6 It was found during the experimental work that agent grade sodiumiodide was not uniform from lot to lot and that a considerable amount ofsuspended impuri-- ties was obtained in some cases. Consequently, toinsure uniform quality ingot products, the raw material sodium iodidewas recrystallized priorto use in the subject process. This is not acritical feature of the invention how? ever. Reagent grade sodium iodidemay be used. for many. applications where optimum quality and lightoutput are not required.

Although theoretical considerations and experimental evidence indicatethat transparent polycrystalline sodium iodide-thallium iodide ingotsare satisfactory scintillation counters, slight process modificationssuch as utilization of a container with a conical-shaped bottom may beused to facilitate the growth of large single crystal ingots by thepresent invention. The use of a conical-shaped bottom enables ingotgrowth to start at the apex of the cone and then grow upward. .Anothermethod of obtaining large single crystal ingots is the use of a seedcrystal in the bas'eof the container. As is known from other ingotgrowth methods, a seed crystal may be used to predetermine theorientation of the growing ingot.

What is claimed is: I. In a process for growing a metal halideingot, thesteps comprising heating raw constituents of such ingot to an elevatedtemperature above the melting points of said raw constituents to form afused melt, maintaining such fused state and substantially continuouslyagitating said fused melt to uniformly distribute the heat through outthe melt, gradually cooling said fused melt, initiating crystallizationin the bottom portion of the melt and growing the ingot upwardly fromsaid bottom portion toward the top of said fused melt.

2. In a process for growing a metal halide ingot, the steps comprisingheating raw constituents of such ingot to an elevated temperature abovethe melting points of said raw constituents to form a fused melt,maintaining such fused state and substantially continuously agitatingsaid fused melt to uniformly distribute the heat throughout the melt,gradually decreasing the temperature and agitation of said fused melt,initiating-crystallization in the bottom portion of the melt and growingthe ingot upwardly from said bottom portion toward the top of said fusedmelt.

3. In a process for growing a metal halide ingot, the steps comprisingheating raw constituents of such ingot to an elevated temperature abovethe melting points of said raw constituents to form a fused melt,introducing heat to the top portion of such melt to maintain the fusedstate, substantially continuously agitating the melt to uniformlydistribute the heat throughout such melt, gradually cooling said fusedmelt, initiating crystallization in the bottom portion of the melt andgrowing the ingot upwardly from said bottom portion toward the top ofsaid fused melt.

4. In a process for growing a metal halide phosphor ingot containing asmall quantity of activator material, the steps comprising heating rawconstituents of such ingot to an elevated temperature above the meltingpoints of said raw constituents to form a fused melt, maintaining suchfused state and substantially continuously agitating said fused meltthereby substantially uniformly distributing the activator material andthe heat throughout the fused melt, gradually cooling said fused melt,initiating crystallization in the bottom portion of the melt and growingthe ingot upwardly from said bottom portion toward the top of said fusedmelt.

5. In a process for growing a metal halide ingot, the steps comprisingheating the raw constituents in a container to an elevated temperatureabove the melting points of said raw constituents to form a fused melt,maintaining such fused state and substantially continuously agitatingsaid fused melt to uniformly distribute the heat 15 throughout the melt,gradually cooling said fused melt,

initiating crystallization in the bottom po tionof the melt, growinggqtuswaraiy from saidbottoni portion tothe top of said fused men,heating'tlie ingot Suffi: cientl'y to melt a large part of the outersurface of'the ingot in direct contact with the container, removing suchmelted large part from the container leaving a vacant space between thecontainer and the ingot except for a remaining unmelted small part ofsuch outer surface, reheating the ingot to substantially uniformtemperature throughout, gradually cooling the ingot'wi'th minimum strain'due to said vacant space, and removing the cooled ingot from thecontainer by severing the remaining small part of the outer'i'ngotsurface in direct contaet with the cont iine 6. In a process for growinga sodium iodide ingot, the steps comprising heating the raw constituentof such ingot to aneleyated temperature above the melting point of saidraw constituent to form a fused melt, maintaining such fused state andsubstantially continuously agitating said fused melt to uniformlydistribute the'heat throughout the melt, gradually cooling said fusedmelt, initiating crystallization in the bottom portion of the melt andgrowing the ingot upwardly from said bottom portion toward the topof'said fused melt.

7. In a process for growing a sodium iodide ingot containing a smallquantity of thallium iodide, the steps comprising heating rawconstituents of such ingot to an elevatedtemperature above the meltingpoints of said raw constituents to form a fused melt, maintaining'suchfused state and substantially continuously agitating said fused meltthereby substantially uniformly distributing the thalliuin iodide,throughout" the sodium iodide and the heat iiielt, gradually coolingsaid fusedr'nelt, initiating 'ct star liz'atioli' in thebot'toi'nportion of the melt and growing the ingot upwardly from's'aid bottomportion toward the top of"saidfusedmelt. 8. In a process for growing asodium iodide ingotcontaininga small quantity of thallium iodide, thesteps comprising heating raw constituents of such ingot abovethe meltingpoints of said raw constituentsto"forrn a fused melt, introducing heatto the top portion of such melt to maintain the fused state,substantially continuously agitating the melt to uniformly distributethe thalli'umiodide throughout the sodium iodide and the heat fusedmelt, gradually cooling said fusedmelt, initiatingcrystallization in thebottom portion of the melt and growing the ingot upwardly from saidbot-tom portion toward the top of said fused melt.

References Cited in the file of this patent UNITED STATES PATENTS1,353,571 Dreibrodt Sept. 2l, 19 20 2,589,310 Tournier Mar. 18, 19522,727,863 Fonda Dec. 20, 1955 2,766,105 Washken 'Oct. 9, 1956 2 3 6 0 2Smith ww OTHER REFERENCES Curran: Luminescense and ScintillationCounter,"

Academic Press Inc., London, pp. 114 to 117 (1953).

7. IN A PROCESS FOR GROWING A SODIUM IODIDE INGOT CONTAINING A SMALLQUANTITY OF THALLIUM IODIDE, THE STEPS COMPRISING HEATING RAWCONSTITUENTS OF SUCH INGOT TO AN ELEVATED TEMPERATURE ABOVE THE MELTINGPOINTS OF SAID RAW CONSTITUENTS TO FORM A FUSED MELT, MAINTAINING SUCHFUSED STATE AND SUBSTANTIALLY CONTINUOUSLY AGITATING SAID FUSED MELTTHEREBY SUBSTANTIALLY UNIFORMLY DISTRIBUTING THE THALLIUM IODIDETHROUGHOUT THE SODIUM IODIDE AND THE HEAT MELT, GRADUALLY COOLING SAIDFUSED MELT, INITIATING CRYSTAL-